Zeiss Distagon T* 18mm f/3.5 ZF (FX) - Review / Test Report - Analysis
Lens Reviews - Nikon / Nikkor (full format)

Distortion

The Zeiss Distagon 18mm f/3.5 produces a moderate degree (~1.8%) of barrel distortion. This may be noticeable but it's actually very good for an ultra wide-angle prime lens. However, there's a bit of a wavy sub-frequency in there so if you want to eliminate the remaining bits of distortion this may not be all that easy anymore.

The chart above has a real-world size of about 120x80cm.

Vignetting

The vignetting characteristic seems to be a real weakness of Zeiss Z-series lenses when used on full format DSLRs. The Distagon shows a fairly extreme deterioration of ~2.1EV at f/3.5 - this will be visible and it's annoying unless you take the key hole effect as an arty aspect. The problem is still very pronounced at f/5.6 (@ ~1.3EV) and acceptable at f/8 and beyond. To be fair - rather high vignetting is fairly typical for an ultra wide angle lens.

We're performing our vignetting analysis based on (uncorrected) JPEGs straight from the camera. The JPG engine of the Nikon D3x features a rather flat gradation curve, thus has a moderate contrast characteristic, resulting in comparatively low vignetting figures - the corresponding Canon figures are roughly 40% higher due to the more aggressive default contrast setting.

MTF (resolution)

The Zeiss has a very good resolution characteristic for such an extreme focal length. The center resolution is very good at f/3.5 and the borders/corners follow on a good(+) level. Stopping down to f/5.6 lifts the center to an excellent and the corners to a very good (just) quality. The peak performance is reached at f/8. Diffraction effects are reducing the resolution from f/11 onwards but the lens is still easily usable at this setting. The field curvature is fairly pronounced but it shouldn't have a big impact at f/5.6 and beyond.

Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure for sharpness. If you want to know more about the MTF50 figures you may check out the corresponding Imatest Explanations

Chromatic Aberrations (CAs)

Chromatic aberrations (color shadows at harsh contrast transitions) are moderate at less than 1.8px on the average at the image borders. This is, again, pretty good for an ultra wide-angle lens although the problem will be visible at times. It is, of course, possible to correct lateral CAs in most modern RAW converters. Many modern Nikon DSLRs already do this for you in-camera if you shoot JPG.




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